The present invention relates to telephone systems and in particular to the apparatus located on the subscriber's premises, be it a simple telephone instrument, a set of multibutton telephone instruments known as a "key system", or a manual or automatic telephone switchboard. Where the owner of the premises desires to prevent long distance calls involving "toll" charges from being initiated from certain of the telephone extensions on his premises, devices known as toll restrictors are used. The present invention specifically relates to an access circuit for use with toll restrictors.
In conventional telephone systems prior to the invention disclosed herein, the availability of toll restriction was mainly limited to users of large switchboards. In one technique, the register in the telephone exchange which receives the number being dialed momentarily reverses the polarity of the telephone line. This signal is detected at the switchboard which then disconnects the caller.
Individual toll restrictors have also been available which could be connected across the telephone line, normally on the subscriber's premises, and would monitor the dial pulses emitted by a rotary dial telephone, and if a "1" or a "0" were dialed, it would interrupt the line. Programmable toll restrictors have also been available which permit calls to certain three-digit prefixes and not to others.
However, prior to the invention disclosed herein, there was no toll restriction arrangement suitable for use with multi-frequency tone-dial telephone instruments, and the toll restriction arrangements which have been available are easily fooled. Among the methods used to fool prior telephone restriction arrangements are the following:
a. Dialing one or two valid digits before the central office is ready. If a valid digit is dialed before the dial tone has been received, it is ignored by the telephone exchange. The toll restrictor has, however, received and recognized the valid digit and assumes a legitimate call is being made. When the telephone exchange is ready and transmits a dial tone, the caller then proceeds to dial a toll number. PA1 b. Placing a local call and then not hanging up. In many cities, if one party does not hang up after the completion of a call which he has either placed or received, he will receive a new dial tone. Since the toll restrictor has already recognized the call as a valid one, the subscriber can proceed to dial a toll call.